Question

Give the reasons why zener diode acts as a reference element in the voltage regulator circuits.

Answer 1

Zener diode acts as a normal pb-np-n junction only that they are specially designed  to have a low and specified Reverse Breakdown Voltage which takes advantage of any reverse voltage applied to it.

Reasons why it acts as a reference element :

1) Zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode.

This value  remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current and the maximum current rating.

2) The ability of the Zener diode to control itself.